Determining location parameters, e.g., position and orientation, of objects in free space has many applications, including catheter tracking, digitizing objects and virtual reality, among others. One method that has become successful in these applications relies on the electromagnetic coupling between a source of magnetic fields and one or more sensors of such fields. Variations include AC and pulsed-DC magnetic field generation and single and multiple axes sensors and field generators. Examples of AC systems utilizing a plurality of field generators and sensors are disclosed by Kuipers in U.S. Pat. No. 3,868,565, Raab in U.S. Pat. No. 4,054,881 and Jones in U.S. Pat. No. 4,737,794, among others.
Conventional systems are generally hindered by inaccuracies resulting from the presence of conductive materials, i.e., electrically-conductive, non-ferromagnetic materials such as aluminum and steel, within the tracking environment. These inaccuracies are caused by the flow of eddy currents within these materials. Eddy currents are caused by the time variation of magnetic fields, such as the magnetic fields produced by the field generators. Each time-varying magnetic field induces a corresponding electric field that, in turn, causes an electric current (eddy current) to flow in the conductive material. These eddy currents, in turn, generate their own (secondary) magnetic fields that can interfere with the sensing of the magnetic fields of the field generators. These secondary magnetic fields can cause inaccuracies in location parameters.
Conventional magnetic field tracking techniques generally ignore the inaccuracies due to secondary magnetic fields caused by eddy currents. However, it would be desirable to eliminate or minimize these inaccuracies, particularly for applications requiring high accuracies and wherein it is difficult, impractical or undesirable to remove conductive materials from the tracking environment. Examples of such applications include the tracking of medical devices within patients' bodies wherein various conductive objects, such as artificial joints and other conductive implants, conductive medical instruments and conductive medical apparatus, such as surgical tables, monitors and other equipment, and the like, are likely to remain in the tracking environment during tracking.
Conventional methods for improving the accuracy of magnetic tracking systems include methods that characterize the environment and apply previously stored corrections. These methods apply corrections based on present position and orientation information (see, e.g., U.S. Pat. Nos. 4,622,644 to Hansen and 4,945,305 to Blood, among others). Other methods include signal generating and processing schemes that allow the induced eddy currents, the source of the inaccuracy, to be eliminated. Such systems utilize pulsed-DC or multi-frequency excitations.
Examples of pulsed-DC systems that utilize a plurality of generating and sensing elements are disclosed by Blood in U.S. Pat. No. 4,945,305 and Anderson in U.S. Pat. No. 5,453,686. The use of pulsed-DC systems reduces the effects of eddy currents by controlling the characteristics of the eddy currents and manipulating the sensed signals so as to minimize their effects, thereby improving accuracy of these systems when conductive materials are present within the tracking environments. A disadvantage of pulsed-DC systems is that they operate only in a time division multiplexed mode. Other drawbacks sometimes include the need for bulky and more complex active sensing devices (compared to sensors used in AC systems). The Blood sensing devices measure field frequencies from DC on up and are thus sensitive to the earth's magnetic field, for which Blood's system must compensate. The broad range of frequency measurement also means that such systems cannot work near medical instruments that operate with large DC magnetic fields, such as magnetic manipulators. The Blood system removes eddy-current-induced inaccuracies by applying a DC excitation signal to a field generator and then curve fitting the decay to extrapolate the final sensed value. The Anderson system eliminates the use of DC sensitive field sensing elements and consequently reduces the complexity of the hardware. His signal processing scheme removes eddy current induced inaccuracies by applying a DC excitation signal to a field generator and integrating the sensed waveform from an AC sensor. This method integrates out the eddy current inaccuracies.
Some conventional DC approaches require an active magnetic sensor that is complex, bulky and has a poor signal-to-noise ratio compared to passive AC magnetic sensors. They are further complicated by the fact that the sensor is sensitive to the earth's magnetic field and processing steps must be included to eliminate the earth's magnetic field. This comes at the expense of the system measurement update rate. Some approaches overcome or reduce some of the disadvantages, but must wait for the eddy currents to die out before determining the value of the field without the deleterious effects of the eddy currents. This too comes at the expense of the system measurement update rate.
Another method for improving accuracy in the presence of conductive materials is disclosed by Rotier in U.S. Pat. No. 4,829,250. The Rotier method is an AC method that includes a plurality of generating and sensing elements and utilizes multi-frequency excitation of the field generator. Eddy current inaccuracies are a function of frequency. This knowledge is utilized by extrapolating to DC a curve fit from a higher frequency to a lower frequency to determine the yaw and pitch angles about a line-of-sight axis, which does not include position.
Another method for improving accuracy in the presence of conductive materials is disclosed by Osadchy et. al. in U.S. Pat. No. 6,147,480. The Osadchy et al. method is an AC method that includes a plurality of generating and sensing elements and utilizes a phase shift detected at the sensing elements and caused by the conductive material. Phase shift differences from a clean baseline (typically zero phase shift) allow the Osadchy et al. system to apply a correction to the measured fields.
Another method of improving accuracy in the presence of conductive materials is disclosed by Ashe in U.S. Pat. No. 6,172,499. The Ashe method is an AC method that includes a plurality of generating and sensing elements and utilizes two excitation frequencies per field generator. The amplitude and phase changes at the two frequencies caused by various distorters at various positions within the tracking volume are stored in a table during manufacture. This table is later accessed during normal operation. Corrections are extracted from the table and applied to the measured fields. The determination of when to use the table is based on the phase shift differences from a clean baseline (typically zero phase shift).
A method further removed from the previously noted techniques for improving accuracy in the presence of conductive materials is disclosed by Elhardt in U.S. Pat. No. 5,347,289. The Elhardt method generates a rotating magnetic field vector of known frequency using a plurality of field generators. Multiple sensors, each with a plurality of sensing elements, are mounted on the object to be tracked. A measurement of the time required for the field vector to pass through a reference point and then through a sensor allows the position of the sensor to be determined. Using multiple sensors mounted in known proximity to one another allows the determination of the orientation of the sensors.